Synthesis of highly dispersed cobalt catalyst for hydroformylation of 1-hexene

Article abstract of DOI:10.1039/c4ra17035k

Highly dispersed Co/SiO₂ catalysts were prepared using an ethylene glycol (EG) modified silica support. The modified SiO₂ support significantly promoted the dispersion of supported cobalt and adding a small amount of Ru increased the reducibility of the highly dispersed cobalt oxide. The Co-Ru/SiO₂ (EG) catalyst exhibited 3 times higher 1-hexene conversion and 18 times higher heptanal yield than the conventional Co/SiO₂ catalyst. This journal is

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ARTICLE TYPE
Synthesis of highly dispersed cobalt catalyst for hydroformylation of 1-
hexene
Bin Wang, Jian-Feng Chen and Yi Zhang*
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
16, 17
Highly dispersed Co/SiO catalysts were prepared by ethylene
to cobalt oxides.
In this study, the highly dispersed Co/SiO catalysts were
2
2
glycol (EG) modified silica support. Modified SiO support
50
2
significantly promoted the dispersion of supported cobalt and
added small amount of Ru increased the reducibility of highly
prepared using a silica support modified by ethylene glycol (EG).
In order to improve the reducibility of highly dispersed cobalt, a
small amount of Ru was added to the Co/SiO catalysts. The
1
0
dispersed cobalt oxide. Co-Ru/SiO (EG) catalyst exhibited 3
2
2
obtained supported cobalt catalysts were tested in slurry phase
hydroformylation of 1ꢀhexene and the promotional effects of EG
pretreatment and added Ru on catalytic performances were
times higher 1-hexene conversion and 18 times higher
5
6
6
7
7
8
8
9
5
0
5
0
5
0
5
0
heptanal yield than conventional Co/SiO catalyst.
2
investigated by XRD, TEM, H ꢀTPR, oxygen titration, H
The hydroformylation is one of the most important syngasꢀrelated
reactions, and the formed aldehydes are valuable final products
and intermediates in the synthesis of bulk chemicals, such as
2
2
chemsorption, XPS and in situ COꢀDRIFT.
Typically, various Co/SiO2 catalysts were prepared by the
incipient wetness impregnation of aqueous of cobalt nitrate onto
1
2
2
3
3
4
4
5
0
5
0
5
0
5
1
alcohols, esters and amines. This reaction could be catalyzed by
SiO support (pore volume: 1.061 mL/g, pore diameter: 6.7 nm,
transition metal complexes, such as Co and Ru, and involves the
addition of hydrogen and carbon monoxide to olefins, yielding
2
2
ꢀ1
specific surface area: 451 m ·g ). The silica supports were
pretreated with EG at room temperature for 1 h by incipient
wetness impregnation method. And then, the samples were dried
2
aldehydes. Cobalt complexes are extensively applied in this
reaction due to its high activity and low cost. All current
commercial processes are based on homogeneous catalysts,
mostly using rhodium. The successful development of a
heterogeneous catalyst for hydroformylation could avoid the
drawbacks of homogeneous catalyst, such as very high pressure,
the catalyst separation and recovery steps.
in air at 393 K for 12 h. A 10 wt% Co/SiO catalyst was prepared
2
as the following steps: a certain amount of Co(NO ) ·6H O was
3 3
2
dissolved in water and then the cobalt nitrate aqueous solution
was impregnated on the support. The obtained samples were
dried in air at 393 K for 12 h and calcined at 673 K for 2 h with a
ꢀ
1
heating rate of 2 K·min . The noble metal Ru promoted Co/SiO
Supported cobalt catalysts were known to be highly active for
2
3
, 4
catalysts were prepared by coꢀimpregnation of cobalt nitrate and
FisherꢀTropsch synthesis.
However, the supported cobalt
ruthenium chloride aqueous solution on SiO support while other
catalysts always exhibit low catalytic activity and selectivity in
2
5, 6
preparing procedures were constant. The loading of Ru promoter
was 1wt%. Before hydroformylation reaction, the catalysts were
reduced by hydrogen at 673 K for 10 h and passivated by 1%
oxygen diluted with nitrogen at room temperature. The prepared
olefin hydroformylation.
The hydroformylation activity of
cobalt catalyst depended on the particle size and the number of
7ꢀ9
active sites. Therefore, cobalt must be in a highly dispersed
form in order to improve the CO insertion reaction and increase
amount of active sites. Synthesis of highly dispersed cobalt
requires strong interaction between the support and the cobalt
precursors, but these strong interactions result in cobalt species
catalysts were denoted as Co/SiO , Co/SiO (EG), CoꢀRu/SiO
2
2
2
and CoꢀRu/SiO2 (EG), where EG indicated that the silicate
supports were pretreated by EG.
1
0
XRD patterns of different catalysts are shown in Fig. 1. As
shown in Fig 1, the catalysts prepared from EG modified silica
support exhibited very weak and broad peaks of cobalt than that
of conventional catalyst. It indicated that the interaction between
supported cobalt and silica support was stronger on EG modified
silica than on conventional silica, resulting in highly dispersed
cobalt oxide which was difficult to be reduced to metal cobalt.
that are difficult to be reduced. Many efforts have been devoted
to improve the dispersion and the reducibility of supported cobalt
11
particles. As our previous report, the concentration, distribution,
and nature of hydroxyl groups (silanols) on the silica surface
played important roles in the dispersion of supported metal on the
12, 13
silica.
Pretreatment of silica by EG was found to increase the
isolated SiꢀOH ratio on the silica surface, leading to the formation
1
4, 15
For the Ru added catalysts, the CoꢀRu/SiO (EG) catalyst still
of smaller particles.
On the other hand, noble metal
2
exhibited very weak and broad peaks of cobalt, indicating the
cobalt particle size of this catalyst was still small due to EG
modification of silica support. It should be mentioned that the
reduced catalysts used here were in the passivated form where the
reduced metallic cobalt particles were covered by a tailorꢀmade
promoters could be reduced at a lower temperature than cobalt
oxides, and shifted the reduction peaks of cobalt species to lower
temperatures. Thus, adding small amounts of noble metals have
been found to significantly promote the reduction degree of
cobalt oxides, presumably by hydrogen spillover from Ru surface
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oxidation layer before being ground in agate mortar for XRD
measurement. Some cobalt oxides from the oxidation layer might
exist besides the originally unreduced cobalt oxide.
The reduction performances for various catalysts were
determined by temperature programmed reduction (TPR), as
35 shown in Fig. 3. Co/SiO catalyst undergoes two typical steps
2
during reduction, which were identified as conversion of Co O
3
4
to CoO followed by conversion of CoO to Co. The peak locating
at higher than 900 K should be the reduction of cobalt silicates,
19
which strongly interacted with support. The first reduction peak
(
d)
c)
4
4
5
5
0
5
0
5
of Co/SiO (EG) located at slightly lower temperature than that of
2
Co/SiO . However, the second reduction peak was very broad
2
(
and located at 650K to 850 K, and the third reduction peak
(above 900 K) became very strong and broad. These results
indicated that the modification of silica support significantly
enhanced the interaction between supported cobalt and silica,
leading to formation of very small and difficult to be reduced
(
(
b)
a)
Co
cobalt particles. For catalyst CoꢀRu/SiO , the reduction peak at
2
CoO
about 434 K should be the reduction of ruthenium species, and
the two reduction peaks for cobalt oxides shifted to lower
temperatures and formed a broad peak in the range of 450ꢀ570 K
indicating that addition of small amount of Ru significantly
improved the reducibility of the obtained catalyst, which could be
2
0
30
40
50
-Theta( )
60
70
80
o
2
5
Fig. 1 XRD patterns of supported cobalt catalysts: (a) Co/SiO
2
, (b)
Co/SiO (EG), (c) CoꢀRu/SiO , and (d) CoꢀRu/SiO (EG).
2
2
2
2
0, 21
explained by hydrogen spillover from Ru to cobalt oxides.
Similar changes caused by Ru addition could also be observed on
TEM images of passivated catalysts were shown in Fig. 2. The
average particle size of the supported Co was also determined by
TEM. The average metallic Co particle size of Co /SiO (EG) and
the catalyst of CoꢀRu/SiO (EG) which exhibited only one broad
2
2
reduction peak located from 400 K to 600 K with several
shoulder peaks, indicating the reduction of various cobalt species
was significantly improved by added Ru.
10
15
20
25
CoꢀRu/SiO (EG) catalyst were 3.4 nm and 3.8 nm, respectively,
2
as compared in Table 1. The dispersion of supported Co was
calculated by the average particle size of Co, as shown in Table 1.
It was found that the catalysts pretreated by EG had smaller
particle sizes and realized almost 3 times higher dispersion than
conventional one, indicating that EG pretreatment could improve
Table 1 Properties of various catalysts.
c
Co particle size Reduction degree
f
g
(nm)
(%)
2
H uptake Dispersion
12ꢀ15
the dispersion of supported Co. As previously reported,
EG
Catalyst
a
b
d
O
2
(ꢁmol/g)
(%)
XRD TEM
TPR
e
pretreatment of SiO decreased the coverage of SiꢀOH on the
titration
2
surface of support and increased the interaction between
supported cobalt particles and silica, resulting in higher supported
cobalt dispersion. The metallic cobalt particles of CoꢀRu/SiO2
Co/SiO
2
7.9
N.A.
9.5
10.1
3.4
39.9
20.8
74.1
63.1
34.1
28.2
17.5
45.9
99.9
9.6
28.1
8.9
Co/SiO2
(EG)
19.2
78.1
61.2
b
catalyst were slightly larger than that of Co/SiO , which should
CoꢀRu/SiO
2
10.7
3.8
2
CoꢀRu/SiO
(EG)
2
be attributed to the fact that added Ru induced cobalt particles
N.A.
25.3
1
8
sintering during the reduction. Therefore, addition of Ru would
a
slightly decrease the dispersion of supported Co. However, for 60 From XRD patterns of passivated catalysts. From TEM images of
c
passivated catalysts. Assuming the noble metal was completely reduced
catalyst CoꢀRu/SiO (EG), the average cobalt particle size was
2
d
at 673 K.
Determined by TPR profiles from 323 K to 673 K. CuO was
still small and almost the same to that of Co/SiO (EG) catalyst. It
e
f
2
used as reference sample. Determined by O
Determined by H
2
titration at 673 K.
2
chemsorption at 373 K. Dispersion was calculated by
g
is considered that smaller cobalt particle size was advantageous
to the adsorption of linearꢀbonded CO and the insertion of CO 65 D=96/d(Co) (%). d(Co) was Co particle size.
during the hydroformylation reaction.
The reduction degrees of the catalysts were calculated
according to the oxygen titration at 673 K and H ꢀTPR from 323
2
K to 673 K, as shown in Table 1. It is shown that the reduction
degrees obtained through oxygen titration and TPR were similar
to each other. EG pretreatment of support made the supported
cobalt oxide difficult to be reduced (19.2% reduction degree),
which was due to the formation of small Co particles on the
surface of EGꢀmodified silica support. For Ru added CoꢀRu/SiO2
7
7
8
0
5
0
and CoꢀRu/SiO (EG) catalysts, the reduction degree was much
2
higher than that of Co/SiO2 and Co/SiO2 (EG) catalysts,
indicating that addition Ru could effectively enhance the
reducibility of cobalt oxide, contributing to forming much more
cobalt active sites. The H2 chemsorption results, as shown in
30
Table 1, proved that the CoꢀRu/SiO (EG) catalyst realized the
2
Fig. 2 TEM images of the prepared catalysts: (a) Co/SiO
2 2
, (b) Co/SiO
maximum of H2 uptake as 99.9 ꢁmol/g, indicating that this
catalyst formed the most active sites in this study, meanwhile, the
(
EG), (c) CoꢀRu/SiO , and (d) CoꢀRu/SiO (EG).
2
2
2
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H uptake of Co/SiO (EG) catalyst is the lowest in Table 1.
2
2
location to Co/SiO due to similar cobalt particle size of Coꢀ
2
Ru/SiO . On the other hand, CoꢀRu/SiO (EG) catalyst exhibited
2
2
45
the strongest adsorption band and obvious blue shift for linearꢀ
type CO peak. These linearꢀtype adsorbed CO molecules at high
(d)
ꢀ
1
wavenumber (2040 cm ) preferred to nonꢀdissociative CO
insertion events, which is advantageous to CO insertion reaction
2
6
and would contribute to hydroformylation reaction. Moreover,
(c)
50 the intensity of linearꢀbonded CO peak for CoꢀRu/SiO (EG) was
2
much stronger than that for CoꢀRu/SiO , because the smaller Co
2
particles and high reduction degree of this catalyst resulted in
much more active sites, contributing to higher activity of 1ꢀ
hexene hydroformylation.
(
b)
a)
(
400 500 600 700 800 900 1000
Temperature(K)
Fig. 3 TPR profiles of the prepared catalysts: (a) Co/SiO
2 2
, (b) Co/SiO
(
EG), (c) CoꢀRu/SiO , and (d) CoꢀRu/SiO (EG).
2
2
5
The XPS study was carried out to determine the chemical
composition and valence state of the elements on the surface of
cobalt oxide. As shown in Fig. 4, from Co 2p core level spectrum
of the catalyst Co/SiO , two peaks located near 780.30 and
2
7
95.73 eV are observed. These two peaks come from the spinꢀ
1
1
2
2
3
3
4
0
5
0
5
0
5
0
orbit splitting and can be attributed to Co 2p and Co 2p1/2
,
3
/2
22
respectively. At the same time, two shakeꢀup satellite peaks,
which arise from the presence of unpaired electrons in valence
orbital, appear in the spectra of Co 2p at about 5ꢀ7 eV higher
energy side from the Co 2p and Co 2p1/2 peaks, indicating the
3
/2
2+
23
55
presence of Co on the surface of catalyst. The similar results
were also observed in Co 2p spectra of the other three catalysts.
It is shown that EG pretreatment caused Co 2p3/2 and Co 2p1/2
peaks to appear at relatively higher binging energy. This result
Fig. 4 XPS spectra of Co 2p core level recorded from prepared catalysts:
2 2 2 2
(a) Co/SiO , (b) Co/SiO (EG), (c) CoꢀRu/SiO , and (d) CoꢀRu/SiO (EG).
2040
indicated a stronger interaction between SiO support and cobalt
0.05
2
oxide species and an electron transfer from cobalt oxide to
support for EGꢀmodified catalysts, contributing to a smaller
14, 24
particle size and low electron density of cobalt oxide.
(
d)
c)
b)
2034
Addition of Ru shifted the Co 2p peaks of Ru added catalysts to
lower binding energy, indicating that the addition of Ru increased
the electron density of supported Co ions.
1934
(
In situ COꢀDRIFT spectra of the reduced catalysts are
2040
2034
2120
(
compared in Fig. 5. For Co/SiO catalyst, an intense peak at 2034
2
ꢀ
1
ꢀ1
cm and weak peaks at 1934, 2117 cm were observed. The
ꢀ
1
2
034 cm peak was assigned to CO adsorbed on cobalt metal in a
2
117
1934
1900 - 1800
(a)
ꢀ
1
linear geometry. The 1934 cm peak was due to the bridgeꢀtype
ꢀ
1
CO on Co sites. The peak at 2117 cm was assigned to the CO
adsorbed on Co (n=2, 3) species. It was shown that the linearꢀ
n+
18
2200
2100
2000
1700
1
Wavenumber(cm )
adsorbed CO peak of Co/SiO (EG) shifted to higher position to
2
ꢀ
1
25
Fig. 5 COꢀDRIFT spectra of various catalysts: (a) Co/SiO
, (b) Co/SiO
2 2
2
040 cm , indicating the small cobalt particles for this catalyst.
60
(EG), (c) CoꢀRu/SiO , and (d) CoꢀRu/SiO (EG).
2
2
On the other hand, the low intensity of all CO adsorption bands
n+
and the high intensity of COꢀCo species on Co/SiO2 (EG)
The prepared catalysts were tested in slurry phase
hydroformylation of 1ꢀhexene under 403 K and 5 MPa, and the
reaction results were compared in Table 2.
indicated that there were more unreduced Co ions on the surface
of Co/SiO2 (EG) catalyst due to the low reduction degree as
compared in Table 1.
As comparison, SiO supported 1wt% Ru catalyst was also
2
For the Ru added catalysts, the CoꢀRu/SiO catalyst exhibited
2
65
tested in 1ꢀhexene hydroformylation. As shown in Table 2, both
Ru/SiO2 catalysts exhibited very low catalytic activity of
hydroformylation, resulting in less than 1% heptanal yield,
stronger linearꢀ and bridgedꢀadsorbed CO peaks than Co/SiO2
due to the higher reduction degree, and very similar adsorbed CO
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indicating that the Ru was not active in hydroformylation of 1ꢀ
On the other hand, smaller particle size of supported cobalt was
hexene. Therefore, in this study, it is considered that the added 25 advantageous for linear type CO adsorption, which was more
Ru only improved the reduction degree of supported cobalt and
did not contribute the catalytic activity of hydroformylation. For
active in CO insertion reaction. Meanwhile, the added Ru
significantly improved the reducibility of highly dispersed cobalt,
contributing to the highest 1ꢀhexene conversion. Based on
mentioned above, the CoꢀRu/SiO2 (EG) catalyst realized the
5
0
5
0
the Co/SiO catalyst, due to large particle size and much less
2
amount of active sites, the conversion of 1ꢀhexene and yield of
heptanal were very low in 1ꢀhexene hydroformylation. 30 highest 1ꢀhexene conversion and yield of heptanal. For the Coꢀ
Meanwhile, even though the Co/SiO (EG) catalyst realized very
Ru/SiO catalyst, the added Ru promoted the reduction degree,
2
2
small cobalt particle size, due to the lowest reduction degree this
catalyst formed the minimum of active sites as compared in Table
contributing to improve the activity of 1ꢀhexene
hydroformylation, resulting in relatively higher 1ꢀhexene
conversion. However, the largest cobalt particle size of Coꢀ
1
1
2
1
, resulting in the lowest heptanal yield in this study.
On the other hand, as is shown in Table 2, the CoꢀRu/SiO (EG) 35 Ru/SiO catalyst was disadvantageous to CO insertion reaction,
2
2
catalyst realized 3 times higher 1ꢀhexene conversion as 93.01%
leading to much lower yield of heptanal than that of CoꢀRu/SiO2
and 18 times higher heptanal yield as 74.09% than conventional
(EG) catalyst. The stability of CoꢀRu/SiO (EG) catalyst in terms
2
Co/SiO catalyst. It is reported that the atoms at the corners and
edges of metal particles are advantageous to CO adsorption in
of heptanal yield is shown in Fig. S1. After 5 reaction entry, the
activity of CoꢀRu/SiO2 (EG) catalyst is still high and do not
2
linear geometry, which benefit the CO insertion during the 40 exhibit obvious decrease, which should be due to the low cobalt
2
6, 27
hydroformylation reaction.
Because the EG modification of
leaching. The strong interaction between supported cobalt
particles and SiO2 prevents the formation of soluble cobalt
species such as cobalt carbonyls which lead to the elution of
silica support significantly improved the dispersion of supported
cobalt and formed smaller cobalt particle, smaller particle size of
CoꢀRu/SiO (EG) catalyst, where the number of atoms at the
16, 28
cobalt.
On the other hand, as shown in Table S1, the BET
2
corners and edges of metal particles was more, was beneficial for 45 surface area of the passivatd and the spent catalysts exhibited
improving the catalytic activity of 1ꢀhexene hydroformylation.
negligible changes, which is benefit for the stability of catalyst.
a
Table 2 Reaction performances of 1-hexene hydroformylation over various catalysts .
b
Selectivity/%
nꢀHeptanal
8.94
d
e
Catalyst
Co/SiO
Conv. /%
c
Yield /%
n/iso
nꢀHexane
0.00
0.00
8.37
4.16
isoꢀHexene
85.37
89.56
32.19
16.18
isoꢀHeptanal
5.69
Total
2
32.69
28.34
50.27
93.01
6.80
14.63
10.44
59.44
79.66
9.87
4.78
2.96
29.88
74.09
0.67
1.75
1.39
2.37
2.34
2.45
2.57
Co/SiO (EG)
CoꢀRu/SiO
CoꢀRu/SiO (EG)
Ru/SiO
Ru/SiO (EG)
2
6.06
41.12
55.84
7.01
4.38
2
18.32
23.82
2.86
2
2
51.22
46.75
38.91
42.57
2
7.37
7.71
2.97
10.68
0.79
a
b
c
Reaction conditions: CO:H
2
=1:1, 403 K, 5 MPa, 1 h, 20 mL of toluene, 13.5 mmol of 1ꢀhexene, catalyst loading=0.1g. Conversion of 1ꢀhexene. Total
d
e
selectivity of nꢀHeptanal and isoꢀHeptanal. Yield of nꢀHeptanal and isoꢀHeptanal. n/iso: nꢀHeptanal/isoꢀHeptanal.
Financial support from the National Natural Science
Foundation of China (Nos. 91334206 and 51174259), Ministry of
Education of People's Republic of China (NCETꢀ13ꢀ0653),
National “863” program of China (No. 2012AA051001 and
2013AA031702) is greatly appreciated.
5
0
Conclusions
A highly dispersed cobalt catalyst was successfully obtained by
pretreating SiO with EG and adding small amount of noble metal
2
7
5
Ru. The CoꢀRu/SiO2 (EG) catalyst realized excellent reaction
performance in 1ꢀhexene hydroformylation, such as 93.01% 1ꢀ
hexene conversion and 74.09% heptanal yield. The promotional
effects of EG pretreatment and Ru addition on the catalytic
performances of the obtained catalysts were investigated by
several characterization methods. The results indicated that
Notes and references
55
60
65
State Key Laboratory of Organic-Inorganic Composites, Research Centre
of the Ministry of Education for High Gravity Engineering and
Technology, Beijing University of Chemical Technology, Beijing 100029,
China. E-mail: yizhang@mail.buct.edu.cn; Fax: +86 10 64423474; Tel:
+86 10 64447274
8
8
9
9
0
5
0
5
pretreatment SiO with EG significantly enhanced the interaction
2
between cobalt precursor and support, resulting in high dispersion
but low reduction degree. Addition of trace amount of Ru could
remarkably improve the reduction degree. High dispersion and
† Electronic Supplementary Information (ESI) available: Experimental
details.
1
A. Neves, M. Calvete, T. Pinho e Melo and M. Pereira, Eur. J. Org.
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2
2
R. Franke, D. Selent and A. Borner, Chem. Rev., 2012, 112, 5675ꢀ
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molecules with strong CꢀO bond, which were benefit for the
catalytic activity in 1ꢀhexene hydroformylation. Therefore, under
the promotional effects of EG and Ru, CoꢀRu/SiO (EG) catalyst
realized the best catalytic activity in 1ꢀhexene conversion and
heptanal yield in this study.
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Acknowledgements
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